How to resolve 'Devices Banned' error when logging into Snapchat on a used iPhone?

There is nothing you can do. The previous owner of the phone violated Snapchats rules and the phone has been permanently banned.

You can try to get your money back from the person who sold you the phone. This is one of the many reasons it's not a good idea to buy a used phone.

If a device is banned by Snapchat, there is nothing you can do. It's likely why they sold the device. Apple has nothing to do with Snapchat or Snapchat bans. If you want to use Snap on an iPhone, buy a new phone, not a used one from someone you likely don't know.

Read this about buying used devices, which also discusses banned devices on Social Media --> The All Too Common SAD Reality of Buying … - Apple Community

Hello ~ Click on below and review this user tip by * lobsterghost1*

The All Too Common SAD Reality of Buying a Used iPad/iPhone*

*Social Media Banned iPhones/iPads:

Social Media Apps, such as Instagram and Snapchat don't like their policies abused. They are very strict about enforcing their policies. When a user does abuse their policies, they can and DO often block not just a users account, but the device itself. And these bans are usually permanent. What does this mean to you if you purchase a used iPhone/iPad? If you buy a device from someone you don't know and one of the issues why they got rid of their device is because it was banned by a Social Media app, then you will have purchased a device which you too can't use the apps which were banned on that device. And this is often without any recourse or way to dispute the ban.

~Katana-San~

That is not a problem that Apple can help you with. Contact SnapChat and if they want you to use their service, they can remove the ban. It most cases they will not do it, but it is worth a shot.